The present invention relates to downhole drilling, and more particularly, to systems and methods for transmitting power to components of a downhole tool string. Downhole sensors, tools, telemetry components and other electronic components continue to increase in both number and complexity in downhole drilling systems. Because these components require power to operate, the need for a reliable energy source to power these downhole components is becoming increasingly important. Constraints imposed by downhole tools and the harsh downhole environment significantly limit options for generating and providing power to downhole components.
Batteries provide one potential energy source to power downhole components. Batteries, however, may be hindered by their inherent finite life and the need for frequent replacement and/or recharging. This may be especially problematic in downhole drilling applications where access to batteries requires stripping and disassembly of the tool string. Battery function may also be impaired by extreme temperatures, pressures, or other conditions found downhole. Many types of batteries may be unable to reliably operate in downhole conditions. Furthermore, batteries may be required everywhere electronic equipment is located downhole, requiring large numbers of batteries and significant time for installation and replacement.
Another approach is to transmit power along the tool string using cables or other transmission media. For example, power may be generated at or near the ground's surface and then transmitted to various downhole components along the tool string. This approach, however, also have its own problems and limitations. Because a tool string may extend 20,000 feet or more into the ground, power transmitted along transmission lines may attenuate to an unacceptable level before it reaches its destination.
Attenuation may occur not only in transmission lines, but in components used to transmit power across tool joints of a tool string. Because a tool string may include many hundreds of sections of drill pipe and a roughly equal number of tool joints, a power signal may attenuate significantly after traveling a relatively short distance along the tool string. In view of the foregoing, what is needed is a system and method for reliably transmitting power to downhole sensors, tools, telemetry components and other electronic components in a downhole drilling system. Ideally, such a system and method would mitigate the problems with signal attenuation which may be present in some power transmission systems. A suitable system and method should also be able to provide reliable operation in extreme temperatures, pressures, and corrosive conditions encountered downhole.
As downhole instrumentation and tools have become increasingly more complex in their composition and versatile in their functionality, the need to transmit power and/or data through tubular tool string components is becoming ever more significant. Real-time logging tools located at a drill bit and/or throughout a tool string require power to operate. Providing power downhole is challenging, but if accomplished it may greatly increase the efficiency of drilling. Data collected by logging tools are even more valuable when they are received at the surface real time.
Many attempts have been made to provide high-speed data transfer or usable power transmission through tool string components. One technology developed involves using inductive couplers to transmit an electric signal across a tool joint. U.S. Pat. No. 2,414,719 to Cloud discloses an inductive coupler positioned within a downhole pipe to transmit a signal to an adjacent pipe.
U.S. Pat. No. 4,785,247 to Meador discloses an apparatus and method for measuring formation parameters by transmitting and receiving electromagnetic signals by antennas disposed in recesses in a tubular housing member and including apparatus for reducing the coupling of electrical noise into the system resulting from conducting elements located adjacent the recesses and housing.
U.S. Pat. No. 4,806,928 to Veneruso describes a downhole tool adapted to be coupled in a pipe string and positioned in a well that is provided with one or more electrical devices cooperatively arranged to receive power from surface power sources or to transmit and/or receive control or data signals from surface equipment. Inner and outer coil assemblies arranged on ferrite cores are arranged on the downhole tool and a suspension cable for electromagnetically coupling the electrical devices to the surface equipment is provided.
U.S. Pat. No. 6,670,880 to Hall also discloses the use of inductive couplers in tool joints to transmit data or power through a tool string. The '880 patent teaches of having the inductive couplers lying in magnetically insulating, electrically conducting troughs. The troughs conduct magnetic flux while preventing resultant eddy currents. U.S. Pat. No. 6,670,880 is herein incorporated by reference for all that it discloses.
U.S. patent application Ser. No. 11/133,905, also to Hall, discloses a tubular component in a downhole tool string with first and second inductive couplers in a first end and third and fourth inductive couplers in a second end. A first conductive medium connects the first and third couplers and a second conductive medium connects the second and fourth couplers. The first and third couplers are independent of the second and fourth couplers. U.S. patent application Ser. No. 11/133,905 is herein incorporated by reference for all that it discloses.